1. Field of the Invention
The invention relates to the field of low voltage power fuses, and in particular concerns a resettable, low voltage current limiting fuse using a current limiting polymer.
2. Prior Art
Interruption of a low voltage circuit advantageously requires a current interruption device that rapidly brings the current flowing through the circuit to zero upon the occurrence of a line fault. A "low" voltage fuse as generally considered herein is of a type employed in power distribution circuits operating up to 600 V.sub.rms. Line faults at these energy levels can cause extensive damage to the circuit components and devices connected to the circuit. To minimize potential damage, fuses are employed with the intent to interrupt current flow quickly, following the onset of fault conditions such as a short circuit.
A typical low voltage current limiting fuse includes: a hollow tubular casing of an electrically insulating material, such as a tubular glass reinforced epoxy casing; a pair of electrical end terminals, such as contact ferrules, closing opposite ends of the tubular casing; at least one fusible element, including reduced cross-sectional arcing regions along its length, electrically coupled between the end terminals, such as silver ribbon or wire.
When the low voltage fuse is subjected to an applied current that exceeds the rated current-carrying capability of the fusible element for a predetermined duration, resistive heating raises the temperature of the fusible element sufficiently to melt it. Tin ("M-effect" material) can be disposed at one or more longitudinally restricted regions along each fusible silver conductor to define relatively lower melting temperature region(s), whereby gaps open at these regions when the fusible element melts.
An electric arc is struck across the gap formed when melting breaks the solid metal conductive path between the terminals. Therefore, one of a plurality of series-connected arcs are formed in the fuse, each having a given resistance. Current through the fuse is interrupted when the sum of the voltages across the individual arcs exceeds the voltage applied to the fuse, stopping the flow of current.
Thus, the current limiting effect is obtained initially by introducing arc resistance in series with the circuit. Over a preferably-short period of time, the arcs that are formed in the gaps of the fusible elements are extinguished as the gaps enlarge. Resistive heating is proportional to the square of the current and will melt the fusible element if the heating exceeds the capacity of the fuse to dissipate heat during a long enough time.
Once the fusible element melts, cutting off the flow of current through the fuse, the fuse must be replaced. This phenomena is particularly problematic on military ships. The replacement of such fuses following electrical faults occurring during battle operations on such vessels can place the ship and its crew at risk. During such an operation it may prove difficult to quickly locate the blown fuse in order to replace it. Furthermore, locating and installing an appropriate replacement fuse may cause critical systems to remain inoperative for an unacceptable period of time.
What is needed is a low voltage current limiting fuse which is resettable or which automatically resets allowing the electrical systems being protected by the fuse to be returned to operation quickly following the occurrence of a fault current.
The present invention provides a resettable low voltage current limiting fuse which can interrupt a fault current and thereafter be reset. Specifically, the invention provides both manually resetting and automatically resetting low voltage current limiting fuse designs.